The semiconductor industry continues to develop lithographic technologies which can print ever smaller integrated circuit dimensions. These systems must have high reliability, cost effective throughput, and reasonable process latitude. The integrated circuit fabrication industry is presently changing over from mercury G-line (436 nm) and I-line (365 nm) exposure sources to 248 nm and 193 nm excimer laser sources. This transition was precipitated by the need for higher lithographic resolution with minimum loss in depth-of-focus.
The demands of the integrated circuit industry will soon exceed the resolution capabilities of 193 nm exposure sources, thus creating a need for a reliable exposure source at a wavelength significantly shorter than 193 nm. An excimer line exists at 157 nm, but optical materials with sufficient transmission at this wavelength and sufficiently high optical quality are difficult to obtain. Therefore, all-reflective imaging systems may be required. An all reflective optical system requires a smaller numerical aperture than the transmissive systems. The loss in resolution caused by the smaller NA can only be made up by reducing the wavelength by a large factor. Thus, a light source in the range of 10 nm is required if the resolution of optical lithography is to be improved beyond that achieved with 193 nm or 157 nm.
The present state of the art in high energy ultraviolet and x-ray sources utilizes plasmas produced by bombarding various target materials with laser beams, electrons or other particles. Solid targets have been used, but the debris created by ablation of the solid target has detrimental effects on various components of a system intended for production line operation. A proposed solution to the debris problem is to use a frozen liquid or frozen gas target so that the debris will not plate out onto the optical equipment. However, none of these systems have proven to be practical for production line operation.
It has been well known for many years that x-rays and high energy ultraviolet radiation could be produced in a plasma pinch operation. In a plasma pinch an electric current is passed through a plasma in one of several possible configuration such that the magnetic field created by the flowing electric current accelerates the electrons and ions in the plasma into a tiny volume with sufficient energy to cause substantial stripping of outer electrons from the ions and a consequent production of x-rays and high energy ultraviolet radiation. Various prior art techniques for generation of high energy radiation from focusing or pinching plasmas are described in the following patents:
J. M. Dawson, xe2x80x9cX-Ray Generator,xe2x80x9d U.S. Pat. No. 3,961,197, Jun. 1, 1976.
T. G. Roberts, et. al., xe2x80x9cIntense, Energetic Electron Beam Assisted X-Ray Generator,xe2x80x9d U.S. Pat. No. 3,969,628, Jul. 13, 1976.
J. H. Lee, xe2x80x9cHypocycloidal Pinch Device,xe2x80x9d U.S. Pat. No. 4,042,848, Aug. 16, 1977.
L. Cartz, et. al., xe2x80x9cLaser Beam Plasma Pinch X-Ray System,xe2x80x9d U.S. Pat. No. 4,504,964, Mar. 12, 1985.
A. Weiss, et. al., xe2x80x9cPlasma Pinch X-Ray Apparatus,xe2x80x9d U.S. Pat. No. 4,536,884, Aug. 20, 1985.
S. Iwamatsu, xe2x80x9cX-Ray Source,xe2x80x9d U.S. Pat. No. 4,538,291, Aug. 27, 1985.
A. Weiss, et. al, xe2x80x9cX-Ray Lithography System,xe2x80x9d U.S. Pat. No. 4,618,971, Oct. 21, 1986.
A. Weiss, et. al., xe2x80x9cPlasma Pinch X-ray Method,xe2x80x9d U.S. Pat. No. 4,633,492, Dec. 30, 1986.
I. Okada, Y. Saitoh, xe2x80x9cX-Ray Source and X-Ray Lithography Method,xe2x80x9d U.S. Pat. No. 4,635,282, Jan. 6, 1987.
R. P. Gupta, et. al., xe2x80x9cMultiple Vacuum Arc Derived Plasma Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 4,751,723, Jun. 14, 1988.
R. P. Gupta, et. al., xe2x80x9cGas Discharge Derived Annular Plasma Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 4,752,946, Jun. 21, 1988.
J. C. Riordan, J. S. Peariman, xe2x80x9cFilter Apparatus for use with an X-Ray Source,xe2x80x9d U.S. Pat. No. 4,837,794, Jun. 6, 1989.
D. A. Hammer, D. H. Kalantar, xe2x80x9cMethod and Apparatus for Microlithography Using X-Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 5,102,776, Apr. 7, 1992.
M. W. McGeoch, xe2x80x9cPlasma X-Ray Source,xe2x80x9d U.S. Pat. No. 5,504,795, Apr. 2, 1996.
Typical prior art plasma focus devices can generate large amounts of radiation suitable for proximity x-ray lithography, but are limited in repetition rate due to large per pulse electrical energy requirements, and short lived internal components. The stored electrical energy requirements for these systems range from 1 kJ to 100 kJ. The repetition rates typically did not exceed a few pulses per second.
What is needed is a production line reliable, simple system for producing high energy ultraviolet and x-radiation which operates at high repetition rates and avoids prior art problems associated with debris formation.
The present invention provides a high energy photon source. A pair of plasma pinch electrodes are located in a vacuum chamber. The chamber contains a working gas which includes a noble buffer gas and an active gas chosen to provide a desired spectral line. A pulse power source provides electrical pulses at voltages high enough to create electrical discharges between the electrodes to produce very high temperature, high density plasma pinches in the working gas providing radiation at the spectral line of the active gas. An external reflection radiation collector-director collects radiation produced in the plasma pinches and directs the radiation in a desired direction. In a preferred embodiment the active gas is lithium vapor and the buffer gas is helium and the radiation-collector is made of or coated with a material possessing high grazing incidence reflectivity. Good choices for the reflector material are molybdenum, palladium, ruthenium, rhodium, gold or tungsten.
In other preferred embodiments the buffer gas is argon and lithium gas is produced by vaporization of solid or liquid lithium located in a hole along the axis of the central electrode of a coaxial electrode configuration. In preferred embodiments, debris is collected on a conical nested debris collector having surfaces aligned with light rays extending out from the pinch site and directed toward the radiation collector-director. The conical nested debris collector and the radiation collector-director are maintained at a temperature in the range of about 400xc2x0 C. which is above the melting point of lithium and substantially below the melting point of tungsten. Both tungsten and lithium vapor will collect on the debris collector but the lithium will evaporate off the debris collector and the collector-director whereas the tungsten will remain permanently on the debris collector and therefore does not collect on and degrade the reflectivity of the radiation collector-director. The reflection radiation collector-director and the conical nested debris collector could be fabricated together as one part or they could be separate parts aligned with each other and the pinch site.
In a preferred embodiment, a unique chamber window is provided designed to transmit EUV light and reflect lower energy light including visible light. This window is preferably a small diameter window comprised of extremely thin material such as silicon or beryllium mounted so as to provide a grazing incident angle incoming beam of about 10xc2x0.
Applicants describe a Dense Plasma Focus (DPF) prototype device constructed by Applicants and their fellow workers as a source for extreme ultraviolet (EUV) lithography employing an all-solid-state pulse power drive. Using the results from a vacuum grating spectrometer combined with measurements with a silicon photo diode, Applicants have found that substantial amounts of radiation within the reflectance band of Mo/Si mirrors can be generated using the 13.5 nm emission line of doubly ionized Lithium. This prototype DPF converts 25 J of stored electrical energy per pulse into approximately 0.76 J of in-band 13.5 nm radiation emitted into 4xcfx80 steradians. The pulse repetition rate performance of this device has been investigated up to its DC power supply limit of 200 Hz. No significant reduction in EUV output per pulse was found up to this repetition rate. At 200 Hz, the measured pulse-to-pulse energy stability was "sgr"=6% and no drop out pulses were observed. The electrical circuit and operation of this prototype DPF device is presented along with a description of several preferred modifications intended to improve stability, efficiency and performance.
The present invention provides a practical implementation of EUV lithography in a reliable, high brightness EUV light source with emission characteristics well matched to the reflection band of the Mo/Si or Mo/Be mirror systems. Since the proposed all-reflective EUV lithography tools are slit scanning based systems, the present invention provides EUV light source with high repetition rate capability.